can you feel it?

They are asking if they can feel any emotion, specifically fear.
During the movie Jonah says "I haven't felt scared since I was 6" and right at the end he says "I feel scared".

Seth also says in the movie "you feel it with her don't you?", I guess you could take this to be any emotion, but I think it could still mean fear because he fears losing her, or fears Seth doing something to her.

Also Jonah obviously does feel things during the movie as we see him get angry with Seth.

I am watching it right now. It's pretty creepy. I like it.
I think when they were talking about feeling "it" they were talking about empathy. One of them just said, "I think that part of us is broken." Seems like they are trying to feel what their victims feel. I think that's what their project is.